A simple presentation about the physiology of mechanical ventilation...

2. PHYSIOLOGY
OF
MECHANICAL VENTILATION
Dr Deepa C MD

3. The Origin of Mechanical Ventilation
“But that life may…be restored to the animal, an opening
must be attempted in the trunk of the trachea, in which a
tube of reed or cane should be put; you will then blow into
this, so that the lung may rise again and the animal take in
air. …and as I do this, and take care that the lung is inflated
in intervals, the motion of the heart and
arteries does not stop…”
Andreas Wesele Vesalius, 1543

4. RESPIRATORY SYSTEM
• Ventilating pump
- Respiratory control centres in the brain
- Connecting tracts and nerves
- Chest wall and respiratory muscles
• Gas-exchange system - Lungs

5. Intrapleural
-10 cm H₂O
Intrapleural
-2.5 cm H₂O

7. POSITIVE PRESSURE VENTILATION
Inflate the lungs by exerting positive pressure
on the airway  forcing the alveoli to expand
during inspiration

8. SPONTANEOUS
Patm Palv ∆P Flow
BREATHING
Inspiration 0 -1 +1 Into lungs
End-inspiration 0 0 0 No flow
Expiration 0 +1 -1 Out of lungs
End-expiration 0 0 0 No flow
POSITIVE PR.
Pinsp Palv ∆P Flow
VENTILATION
Inspiration 20 0 +20 Into lungs
End-inspiration 20 20 0 No flow
Expiration 0 20 -20 Out of lungs
End-expiration 0 0 0 No flow

9. TRANSMURAL PRESSURE
“Pressure across the wall”
Difference in pressure between the inside (Pi) and the
outside (Po) of any structure
EQUILIBRIUM VOLUME of a structure
The volume it contains when the transmural pressure
(Pi - Po) is zero

11. LUNG COMPLIANCE (DISTENSIBILITY)
- change in volume per unit change in pressure - ∆V/∆P
Static compliance = Tidal volume
Pplat – PEEP
(measured when there is no air flow)
Dynamic compliance = Tidal volume
Ppeak – PEEP
(measured when air flow is present)

13. ELASTANCE
The retractive (recoil) force generated by the
recoil of an elastic structure
Inversely related to compliance
A less compliant lung has higher elastance

14. RESISTANCE
Resistance = ∆P/Flow
Inversely proportional to R⁴
WORK OF BREATHING - work performed by
the respiratory muscles in stretching the
elastic tissues of the chest wall and lungs
(elastic work – 65%), moving inelastic
tissues(7%) and moving air through the
respiratory passages(28%)

15. Pplat & Ppeak
• PLATEAU PRESSURE - is the pressure needed to
maintain lung inflation in the absence of air flow
• Measured by occluding the ventilator 3-5 sec at
the end of inspiration
• PEAK INSPIRATORY PRESSURE - Pressure used to
deliver the tidal volume by overcoming non-
elastic (airways) and elastic (lung parenchyma)
resistance

17. PENDELLUFT EFFECT

19. A. Increased Airway B. Decreased Compliance
Resistance of Lungs & Chest Wall
PIP PIP
Pplat
Pplat

21. Tidal Volume = Inspiratory Flow x Inspiratory Time
(ml) (ml/sec) (sec)
∆ Pressure = Inspiratory Flow x Resistance
∆ Pressure = Ppeak – Pplat

22. DEAD SPACE & SHUNT
DEAD SPACE – wasted ventilation
(no gas exchange due to absent perfusion)
eg.; pulmonary embolism
SHUNT – wasted perfusion
eg.; atelectatic segment, one-lung
ventilation

23. POSITIVE END EXPIRATORY
PRESSURE (PEEP)
Increases the end expiratory or baseline airway
pressure to a value greater than atmospheric pr. on
ventilator manometer
INDICATIONS
• Intrapulmonary shunt and refractory hypoxemia
• Decreased FRC and lung compliance
• Useful in maintaining pulmonary function in non-
cardiogenic pulmonary edema, especially ARDS

24. PHYSIOLOGY OF PEEP
Opens up collapsed alveoli and prevents alveolar
collapse during exhalation
PEEP
Decreases alveolar distending pressure
Increases FRC by alveolar recruitment
Improves ventilation
Increases V/Q, improves oxygenation,
decreases work of breathing

25. • Prevents early airway closure and alveolar
collapse at the end of expiration
• Increases(and normalizes) the functional
residual capacity (FRC) of the lungs
• Facilitates better oxygenation
Note: PEEP is intended to improve
oxygenation, not to provide ventilation, which
is the movement of air into the lungs followed
by exhalation

26. Increases surface area for gas exchange by opening the
collapsed alveoli
Translocation of fluid to peribroncheal region in pulm
edema

27. DISEASES WHERE PEEP IS USED
• ARDS/ALI
• Cardiogenic pulmonary edema
• Unilateral lung ventilation & postop hypoxemia
• COPD
COMPLICATIONS ASSOCIATED WITH PEEP
• Barotrauma
• Diminish cardiac output
• Regional hypoperfusion
• Augmentation of I.C.P.
• Paradoxical hypoxemia
• Hypercapnoea and respiratory acidosis

28. SPONTANEOUS BREATHING

29. POSITIVE PRESSURE BREATHS

33. Indications for mechanical ventilation
• Ventilatory failure
• Oxygenation failure
• Excessive ventilatory workload
• Impending respiratory failure

34. VENTILATORY FAILURE
 Drug overdose
 Spinal cord injury
 Head injury & stroke
 Neuromuscular dysfunction
 Sleep disorders
 Acute airflow obstruction
 Chest trauma
 Postoperative – thoracic & upper abdominal
 Electrolyte imbalance
 General anaesthesia

35. OXYGENATION FAILURE &
INCREASED VENTILATORY WORKLOAD
 Acute lung injury/ARDS
 Acute severe airflow obstruction
 Dead space ventilation
 Shunts
 Congenital heart diseases
 Shock
 High metabolic rate & Obesity
 General anaesthesia & Postop

36. GOALS OF MECHANICAL VENTILATION
 Maintain patient comfort
 Allow a normal, spontaneous breathing
pattern whenever possible
 Maintain a PaCO₂ between 35 - 45 mmHg
 Maintain a PaO₂ sufficient to meet cellular
O₂ demands but avoid oxygen toxicity
 Avoid acid-base and electrolyte imbalances
 Avoid respiratory muscle fatigue and atrophy

37. EFFECTS OF
POSITIVE PRESSURE VENTILATION
 Decrease in venous return
 Decrease in cardiac output
 Decrease in pulmonary capillary blood flow
 Increase in pulmonary vascular resistance
 Increase in central venous pressure
 Increased intracerebral venous pressure
 Decreased CSF absorption
 Increased intraabdominal pressure

38. Contd…
 Increased vasopressin secretion
 Decreased GFR & urine output
 Increased fluid retention
 Paradoxical fall in PaO₂
 Barotrauma/volutrauma
 Ventilator-associated pneumonia
 Oxygen toxicity
 Prolonged intubation – airway problems
 Pressure sores
 Ventilator dependence

39. AUTO-PEEP or INTRINSIC PEEP
Airflow obstruction
Low I:E ratios
Increased respiratory rate
Low flows
Leads to dynamic hyperinflation

40. AUTO-PEEP or INTRINSIC PEEP

41. DYNAMIC HYPERINFLATION
Expiratory flow obstruction
Increased rate
Decreased expiratory time

42. MONITORING
 Physical examination for all body
systems focusing on the resp system
 Assess the patient for decreased cardiac
output
 Administer a sedative as ordered to relax
the patient
 Evaluate the settings of mechanical
ventilator
 Ensure patient safety ( side rails )..

43. Thank you